An aircraft has a plurality of moving parts such as a rudder surface and a thrust deflection nozzle. When the aircraft having the moving parts is linearly controlled, robustness may be insufficient. In order to improve the robustness, an application of nonlinear optimal control is considered. In order to achieve the nonlinear optimal control, it is necessary to solve the Hamilton-Jacobi's equation which is nonlinear expansion of Riccati equation. As one of solutions of the Hamilton-Jacobi equation, a stable manifold method as disclosed in Non Patent Literature 1 and Non Patent Literature 2 is known.